Apparatus for electrical well-logging



April-2, 1957 J. E. wALsTRoM 2,787,758

APPIARATUS FOR ELECTRICAL WELL-LOGGING 'original Filed March 13, 195o l 4 sheets-sheet 1 April .2? 1957 J. E. wALsTRoM 787,758

APPARATUS FOR ELECTRICAL WELL--Loscm original Filed Maren 1:5, 195o" 4 Sheets-Sheet 2 wir 'im IU -INVENTOR John E; Walstrom 1957 J. E. WALSTROM 2,787,758

AHPARATUS FOR ELECTRICAL WELL-LOGGING Original Filed March l5, 1950 4 Sheets-Sheet 3 lhNvENToR I John Walsfrom ATTORNEYS AP 2, 1957 J. E. wALsTRoM APPARATUS FOR ELECTRICAL WELL-LOGGING 4 Sheets-Sheet 4 Original Filed March 13, 1950 INVENTOR John E. Walsfrom cgb# fgzg ATTRNEYS nited States Patent O 2,787,758 APPARATUS Fon ELnCraICAL WELL-LOGGING Original application March 13, 1950, Serial No. .1149,297.

Divided and this application June 8, 1953, Serial No. 360,309

7 Claims. (Cl. 324-1) The present invention relates to an improved apparatus for electrically logging uncased wells or bore holes, and more particularly relates to an improved apparatus for qualitatively investigating the nature of the strata traversed by an uncased well or bore hole. This is a division of my copending application Serial No. 149,297, entitled Method for Electrical Well-Logging, tiled March 13, 1950, now Patent No. 2,694,179.

It has long been `appreciated in the art of well logging that the simplest and easiest means of investigating the physical nature of the strata traversed by an uncased well or bore hole is the electrical method. In the usual methods employed heretofore, these electrical determinations have most frequently taken the form of one or more resistivity measurements and a single measurement of the spontaneous potential existing in the well bore. These measurements are ordinarily continuously recorded throughout the entire length yof the uncased well bore `and in combination with other lithological information, may enable the user to better identify the uid content of the formation.

The resistivity measurements in conjunction with other information may often serve to differentiate between the various strata which contain salt water and those which contain petroleum since the presence of petroleum is indicated by increased resistivity. However, increased resistivity alone may also be due to low porosity of the formations or fresh water or both. Thus, a high value of resistivity does not necessarily indicate the presence of petroleum-bearing strata. Accordingly, it is necessary to have further lithological information before the increased resistivity in one stratum, as compared to another, can be tentatively identified as `due to the presence of petroleum. The phenomenon which assists in this further identification of the nature of the strata is the natural, or spontaneous potential, frequently abbreviated as s. p., existing in the well bore and which may be recorded by the same array of electrodes utilized in the resistivity measurements. These potential measurements are made with only the natural, or spontaneous, potentials generated in the bore hole acting as the current producing source. While the exact nature and source of these potentials have not been fully identified, it is well known that they do exist and have been shown experimentally to be at least partly the product of electrochemical action, due to the interaction of clay-like materials such as shale, fresh-water drilling mud, and salt-water saturated sand.

In the case of a log made in a development well, it may often be that the resistivity curves and the potential curves are satisfactory for identifying the liuid content of the strata and, in a qualitative manner, the permeability or porosity of the formations. However, in a log made in an exploratory well, where little lithological information is available, the formations so identified by the electric log as being permeable, or porous, have sometimes turned out to be formed of impervious materials, such as non-fractured chert, limestone, or compacted shale. Accordingly, it has been found necessary in most instances l 2,787,758 Patented Apr. 2, i957 V'ce to investigate the well formations by further and more exhaustive means, such as formation testing and coring. However, due to the expense of these last-mentioned operations, they are employed as little as possible.

Accordingly, there has long been a need for a method for better determining the presence of porous or permeable strata or, conversely stated, the absence Iof nonporous strata, traversed by a well bore by means of electrical logging apparatus, since the operation of such equipment is economical to employ and easily applied as compared to other formation evaluation techniques.

In certain of its broader aspects, the novel method of electric logging, in accordance with the present invention, comprises the step of measuring or otherwise determining qualitatively the internal resistance characteristic of the potential generating source of the spontaneous or natural currents generated in a bore hole. In addition, the present invention contemplates novel apparatus for measuring the internal resistance characteristic of the potential generating source of the spontaneous currents existing in a well or bore hole.

While some of the apparatus proposed for the practice of the present invention is similar to or even identical with that conventionally employed in electrical logging, the novelty of the present invention resides in the method or sequence of operations involving the determination of electrical characteristics that were hitherto unknown and unappreciated as indicative of formation characteristics. However, in addition to the conventional apparatus which may be adapted to practice the method lof this invention, novel arrangements for the carrying out of the subject method are disclosed which may be adapted for conducting conventional electric logging procedures.

lt is therefore an object of the present invention to provide an improved method of qualitatively `determining the nature of strata by the practice of novel operations and measurements which may be made by well known electrical logging equipment.

It is a further object of the present invention to provide a less expensive method for qualitatively determining the porosity or non-porosity of strata traversed by a well bore.

A still further object of the invention is to provide improved electrical apparatus for qualitatively investigating the porosity and liuid content of earth formations in a well.

Another robject of the invention is to provide an improved method for identifying strata which may be commercially important in tne production `of petroleum.

Further objects and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings which form an integral part of the present specification.

In the drawings,

Fig. l is a schematic representation of a well logging apparatus of the single-electrode type which may be used in the practice of the invention.

IFig. 2 is a schematic representation of a 2-electrode apparatus which may be used in the practice of the method disclosed in the present invention.

Fig. 3 is a schematic representation of a well-logging arrangement, providing two circuits, each similar to the circuit of Fig. l, for practicing the invention.

-Fig. 4 is an alternative well-logging arrangement providing measurements similar to those made by the circuits of Fig. 3 but utilizing only one insulated lead.

Fig. 5 is a novel electrical well-logging apparatus which may be employed in the practice of the method disclosed by this specification and which provides a multiplicity of measurements and also eliminates the resistance of the cable leads.

VFig. 6 is an alternative arrangement of an electric welllogging apparatus, similar to that shown in Fig. 5.

Fig. 7 is a graphical representation of the manner in which the internal resistance characteristic of a conventional cell, similar to the cells produced in the earth formations traversed by a bore hole, may be evaluated from data giving the current iiowing at various external resistances. Y

Fig. 3 is an elementary circuit diagram of the potential generating and measuring circuit utilized to establish the values plotted in Fig. 7'.

Fig. 9 is a schematic representation of the earth formations traversed by a bore hole, as interpreted through the use of a typical electric log produced by the practice of the present invention.

Referring now to the drawings,y and particularly to Fig. l, there is shown a conventional single electrode circuit for measuring the bore hole potential, which ffor simplicity ofexplanation is shown in a simplied, schematie form. AsV there shown, ajhore hole. 1 produced' by conventional means, such as rotary drilling, traverses a series ofearth strata 2. Strata 2, in general, may be considered to include formations such as shale 3', and sandstone d, which may be saturated with Ysalt water. According to the present theory of generation ofnatural, or spontancous potentials, the presence of a fresh water mud in borehole 1 will establish an electric current iieldapproximately accordance with the current lines 6, the smallest of which is centered Vabout the common intersection of the sand, shale and bore hole. In conventional recording of the potential cur-vein electric welllogging, the potential fieldin the drilling fluid as established by the flow or electric current asdepicted is detected at the earths surface by a circuit such as that provided by a down-holel electrode 7 connected by an insulated cable 8. to a record-ing alvanometerl Q-W-hich may be shunted by a variable shunt resistor lil. The op.- 'PQsite terminal of galvanometer is connected to asurfBCf ground-electrode. l?. through a potentiometer circuit i2, which may comprise. a battery 13 and a variable potentiometer 1d, and a variable. series. resistor 15..

In conventional practice, it, has. been customaryV torecord the output or" galvanometer 9 in asingle upward pass or traverse of the down-hole,- electrode 7 from the bottom of well bore l to theI surface of the groundA or to the deepest metal casing placed in the bore hole. lin. this single-pass recording operation, variable potentiometer 14 and shunting resistor 10 are adjusted so that the-maxi- :mumY deflections of galvanometer 95' may be recorded satisfactorily on the record strip. A full scale value` offpotential of i) millivolts,is4 ordinarily found plactical and satisfactory. VSeries resistor isl normally provided. to insure a very small" current lowby proyidinga very high resistance in the series circuit'.

The conventional electric log potential records merely represent the potentials present in the bore hole, as moditiedf by the drilling uid present, and in no Way represented an` absolute` valueofV the electromotive` force of the potential cell.

As distinguished from this prior practice ofonlyv recording during a single'passage ofelectrode 7 through. the well bore i, l have foundthat, by. providing a range of resistance valnestor rcsiStQrlSswhichisin. series with galvanometer 9;, and. then preselecting the,A valuesA ofV resistance for resistor l5, it is possible. to tovideramcans of obtaining at least two. separate and. dierentpoternial curves under diterent conditions of, external.; load, resistane- YThese distinct. potential. curves. may then. bc used to qualitatively evaluate or determine the hitherto overlooked internal resistance. characteristic 0f the natural current generating sources. This may be accomplished by suitably changing the.- totalcircuit current flowingfin the measuring circuit andA at the same time preterably, but not necessarily, adjusting; the galvanometer sensitivity to itsy originalvalueby properly adjustingv re,-

sistor lll'. In this way two potential*curves.ofdiiferent,AV

A but comparable, deilection magnitudes may be obtained. By then comparing these distinct potential curves, according to the procedure which will be described in further detail below, a novel method of further and more fully investigating the physical characteristics of the earth .rata has been provided since the internal resistance characteristic of the generating source is dependent upon the nature of the strata which form a part of the generating cell, or source, that produces these natural potentials. This is believed to be due to the fact that the porosity of the strata determines to a large extent the amount of conductive, or non-conductive, fluids present in the formation.

Since the operation of the circuit shown in Fig. l will be best understood in connection with the explanation of the electric well logs shown in Fig. 9 described hereinafter as well as the method of evaluating these logs as described in conjunction with Figs. 7 and S, the details of a preferred method of operating the apparatus of Fig. l will be discussed in conjunction with those ii'gures.

Referring now to the alternative arrangement of welllogg-ing apparatus schematically shown in Fig. Z, it will be noted that the apparatus is substantially similar to that shown in Fig. l with the exception that the left-hand sideI or potentiometer lilisconnected to another downhole electrode i6 through insulated conductor l? instead of to ground-electrode il. Electrode i6' is preferably spaced longitudinally from electrode 7 in such a manner that the intervening potentials existing in the bore hole 1 be detectedbetween electrodes 7 and 16. The circuit elements of the arrangement shownV in Fig. 2 are identitied by the corresponding numbers to those shown in Fig. l and operate in the same manner. The spacing of electrodes 'Z and 16 may be` of the order of from l to 5 feet. e

Fig. 3 shows an alternative, arrangement employing two circuits similar to that shown in Fig. l. lt will be noted thatV a. separate indicating, or recording, circuit is connected to down-hole electrode i6, Whichin` the present instance is connected through insulated leadV 118 to a second galvanometer i9 which is shunted by a resistor 20. rlhe opposite. side of galvanometer it@ may be connected to a separate ground-engaging surface electrode 2l.- through a potentiometer circuit 22 and a series resistor 25. The, purpose of the second galvanometer circuit is, to provide; a. means. for' indicating, or recording, the well bore potentials; under twoV different valuesY of' external circuit. resistance by a single passage: of the e ywell logging electrodes through. the well bore l.

Fig. 4 shows another alternative; wellflogging arrangement similar toy that: shown in Eig.A 3,' in which; a single electrode 7 is arranged to bepassedthoughebore hole' l byV means` of cable: S- and alternately connected. to the. indicating.l circuits provided by a. rstgalvanometer 9 anda second galvanometer 25.9 by means, of a commutator switch 26 which is adapted to periodically oscillate; between contactors 27 and 28. By the present. arrangement, it is possible to record simultaneously and; with sullicient accuracy the current flow in the; two galvanometer circuitsconnected' to instruments 9 and 3.19 under different resistance loading conditions by the passage of. only a single electrode4 7 throu-ghi bore hole 1. Such; a method as. thatA shown inthe present` arrangement allows' two curves toberecorded using, onlysingle lead-in:V those cases, infwhich a minimum. number of leads, or conductors, must. be. employed.

Referring now to. the apparatusV shown-in; Figi 5, yit will be noted that. a. novelV well-.logging device for. practicing. the method ot the present invention has been provided whichY is. adapted to. be lowered. into.y therbore, hole. l by a multiconductor cable which may include. the. ve leadsy 33,. 351, 321,33 and 34: which are connected to; theconztrol and indicating. apparatus. ln general,` this-well-logging tool comprises.. a. head. 35i which is. adapten. to. bescrewed onto, the upper portion of a hollow. cylinder member.A 3.6.

by means ofl screw threads. 37. The. lower portionofcyl.-

ayzsmssr inder member 36 is sealed by end cap 38 which issecu'red to cylinder 36 by screw threads 39. The outer wall of cylinder 36 is preferably provided with an insulating sheath 4i? upon which may be mounted a pair of circular electrode rings 41 and 42. Electrodes 41 and 42 may be so positioned on sheath 40 that the desired spacing of the electrodes may be obtained for detecting the spontaneous potentials present in the well between a desired interval.

ln order to detect the natural potentials existing between electrodes 4i and 42 under varying load conditions and to provide a measuring circuit for the current generated by these natural potentials, provision has been made to operate stepping motor M-2 which is adapted to be energized through conductor leads and supplied by a suitable source of electric power, such as generator 29, through switch 43. By the operation of motor M-2, contacter arm 44, which is mounted on the motor shaft, may be moved to any of the contact points connected to the plurality of resistors 45 that may be of preselected, stepped values and which have their opposite ends connected to electrode 42. The opposite side of movable contacter 44 is adapted to be connected to the field coil 46 of an alternating current generator, or alternator, designated generally as 47, which may be continuously driven by motor M-l. Motor M-1 may be energized through lines 31 and 32 by the same source of power used for motor Nif-2. The opposite side of field coil 46 is connected to well electrode 41 so that the magnetic field intensity in tield coil 46 will be proportional to the current liowing between electrodes 41 and 42. By this arrangement the current produced by the well bore potential acts to produce an alternating potential. The output of armature 4S may be connected to a pair of slip rings 49 to provide a current flow through lines 33 and 34 which may be utilized to operate galvanometer 9 with its potentiometer 12, 13, 14, in much the same manner as the directly connected current electrodes used in Figs. 1 to 4. For the purpose of converting the alternating current generated in alternator 47 to a direct current, a rectifier 50 may be provided between the connection to one of the slip rings 49 and cable lead 34. An important advantage of this is that the minimum value of external circuit resistance which includes the resistance of the conductor connected to the down-hole electrode, may be reduced to almost zero. Since the resistance of the down-hole conductor is generally dependent upon length in certain cases, it may be very important to exclude as much of this resistance as possible.

With reference now to an alternative Well logging apparatus shown in Fig. 6, it will be noted that four potential electrodes, S1, 52, 53, and 54, have been positioned on insulating sheath 40 of a well logging tool similar to that shown in Fig. 5. In this embodiment, electrodes 5l and 52 are adapted to be connected through a pair of cable leads S5 and 56, respectively, to a galvanometer circuit 9 on the earths surface in a conventional manner. in this arrangement the resistance of the cable leads S1 and 52 are unavoidably included as a part of the total circuit resistance. However, in this circuit the cable lead resistance is not detrimental since a very high external circuit resistance is desirable. Electrodes 53 and 54, on the other hand, are arranged to be connected directly to an alternator, or A. C. generator, field coil 57 to provide an alternating current output from armature 53 which is proportional to the potential between. electrodes 53 and 54. in this circuit a very low external circuit resistance is desirable and the cable lead resistance is completely excluded. By this arrangement, two circuits having quite diiferent external circuit resistances may be obtained. Motor M-l which drives armature 58 is arranged to be driven by power supplied through lines 59 and 60, which are connected to a suitable source of electric power, such as generator 69, at the earths surface. However, as distinguished from the arrangement shown in Fig. 5, the output of armature 58 is applied from slip rings 61 to a high impedance/transformer 62 whose secondary coil 63 is connected to an amplifier 64 and the amplified A. C. output potential may then be converted to a D. C. potential by rectier 65. In this manner only D. C. is transmitted over the cable leads, since the output of rectifier 65 may be connected to Igalvanometer 66 which is located on the surface by means of leads 67 and 68. It will be noted that rectifier 65 and amplier 64 are likewise arranged to be energized through power supply lines 59 and 60. The arrangement shown in Fig. 6 thus provides two measurements of well bore potential, one measurement at a relatively high external circuit resistance and the other measurement at a relatively low external circuit resistance.

Reference is now made to the graph shown in Fig. 7 and the elementary diagram shown in Fig. 8 which are representative of the values to be measured by the apparatus described in the foregoing gures. As shown,

particularly in Fig. 8, the ammeter I is connected in series with a variable resistance Re to a source of potential E, represented as a battery which has a series resistance Rz'. In the measurement of current flow through an external resistance Re, it can be shown that the reciprocal of the current (l/I) will vary directly as the sum of Re-i-Ri. For this reason, when the internal resistance of the generating source E represented by Ri cannot be directly measured, it can be shown as in Fig. 7 that the external resistance may be plotted against the reciprocal of the current for that particular value of external resistance to establish a point A. Then, by varying the external resistance by altering the value of Re, it will be apparent that a new value of Re and l/I may be plotted as point B. In the same manner, points C and D may likewise be obtained if they are so desired and these points interconnected by the straight line L which may be extrapolated to the point where it crosses the X axis. In this manner, it may be demonstrated that the intercept on the X axis, which is represented by the distance from Re=0 to the point where line L crosses the X axis gives the internal resistance Ri of the potential source.

In accordance with this invention, there is obtained a determination of the magnitude of the internal, or selfresistance of the formations which indicates qualitatively the degree of porosity, or lack of porosity, of the earth strata. Accordingly, in the apparatus shown in Figs. l through 6, the primary purpose of the variable resistance in the galvanometer circuit is to provide a means for obtaining at least two distinct conditions of current loading to permit an analysis of the internal resistance characteristics of the spontaneous potential sources originating in the formations and created when a well bore traverses geological strata of diterent physical properties, such as a sand-.shale series.

As a representation of the information which may be obtained by the process disclosed by this invention, a typical well log is shown in Fig. 9 in which, at the left side, the strata traversed by the bore hole are labeled in accordance with the lithological nature of the materials as determined by coring or sidewall sampling, while at the right there is shown a typical electric log, or current proile, of these formations. Curve M will be recognized at the usual resistivity log which may be obtained by any conventional resistivity measurement method. Curve N is, in general, representative of the typical potential curve such as is made in any conventional electric logging operation, but produced at only one, and usually a high, value of external circuit resistance. It will be particularly noted in this regard that at the HOO-foot interval the resistivity curve shows a high resistivity and, likewise, the potential curve N shows a high potential. However, actual side-wall sampling or coring proved that the formation was tight and impervious chert, and not a porous of permeable formation, as may be construed from the appearganarse ance ofthe conventional log. However, I have found that by .noting the external resistance when curve N was made and then changing vthe external circuit resistance, for instance, to a very low value, a further potential curve P could he run Vin which the potential curve will be `noted to be substantially Vdifferent than the original curve N. A comparison and rstudy of the dierences in the two curves N and P permits a qualitative evaluation of the internal characteristics of the -media traversed and thereby a better evaluation of the physical characteristics of the strata and more particularly the porosity and fluid content thereof.

In vthe operation of the apparatus shown in Figs. Vl and V2, in accordance with the teaching of this invention, it

will be readily Vapparent to :those skilled in the art, that potential curve N, as shown in Fig. 9, may be produced by first selecting a very high resistance value for variable resistor i5, adjusting the galvanometer sensitivity and then traversing the well bore with electrode 7, or in the case of Fig. 2 both electrodes 7 and 16. Curve P would then be developed by selecting another value, for instance very low, of external resistance by means of resistor 15, again adjusting the galvanometer sensitivity to the same scale value and then repeating the traverse of bore 1 with the electrodes and recording the values indicated by galvanometer 9. It will, of course, be obvious, that if a particular stratum traversed by the well bore appears to be of particular interest, thenseveral diterent values of resistance may be selected, and the current or potential values measured by galvanometer 9 may be recorded. In this way, discrete segments or points of the curve may be analyzed in accordance with the graph shown in Fig. 7, by plotting the reciprocal of the current versus the external resistance values to determine the apparent internal resistance of the earth system at the horizon in question.

Alternative to the arrangements shown in Figs. 1 and 2, the apparatus shown in Fig. 3 may be utilized to simultaneously produce curves N and P by selecting very diierent Values for the series resistors 15 and 25, and simultaneously recording the potential, or current, indications on galvanometers 9-and 19.

A still further alternative to the arrangement in Fig. 3 is provided by the apparatus shown in Fig. 4 for simultaneously recording curves N and P by withdrawing electrode 7 from the bottom of the well bore to the surface of the earth and periodically, or cyclically, moving contactor 26 between stationary contactors 27 Vand 58 to successively energize galvanometers 9 and 19, which are connected to surface electrodes through series resistors 15 and 2S, which may be set at diierent values of resistance. This system thus utilizes only one insulated conductor rather than two as in Fig. 3.

In regard to the operation of the novel apparatus as shown in Fig. 5 which may be used in the practice of the method disclosed by this invention, it willbe noted that potential sampling electrodes 41 and 42 are arranged to traverse the bore hole 1 when the apparatus'is lowered or raised by means of the insulated cable, which includes leads 3i?, 3i, 32, 33 and 34. Por the purpose of providing a different circuit resistance for each traverse of the bore hole by the logging tool, the plurality of resistors 45 are arranged to be of different resistance values so that the total circuit resistance between electrodes 41 and 4Z may be changed at will from almost zero resistance to a very highvalue. The placing of the resistors in the downhole instrument provides a measuring circuit capable of excluding the resistance of the cable leads and thus obtaining a much lower value of external circuit resistance than is possible with the logging apparatus shown in Figs. l through 4.

As stated hereinbefore, movable contacter 44 is adapted to engage the ends of any of the resistors 4S by operation of stepping motor M-Z which is energized through lines 30 and 31. With `movable contactor 44 connected to one of the Anesistanrces 45, the total current flowing in field coil 46 of alternator 47 will vary in accordance with the poentialexisting between electrodes 41 and d2. Then, with varmature 48 being rotated 'at a constant speed by drive motorV M-l, an alternating current potential will be impressed upon slip rings 49 which is proportional to the current in the potential responsive circuit comprising coil 46 and one -of the resistors d5. The alternating current output of alternator 47 may then be applied to the galvanometer '.9 at the earths surface through rectiiier Sil and the Vcable leads 33 and 34. It will be'apparent that with the apparatus shown `in Fig. 5 as many traverses may be made of the bore hole as there 'are diierent external resistances yprovided by resistors 45. it should again be noted that by placing the resistors in the down-hole instrument the total circuit resistance may be made much lower in value than that of Va single cable lead. In order to reduce the total circuit resistance even lower it may be desirable to employlarger sizes of potential electrodes to contact the drilling fluid in the well bore.

With reference to the operation of the appara-tus shown in Fig. 6, it will be noted that curves i and N may lbe recorded vsirmiltaneously by the provision of two separate pairs of potential sampling electrodes adapted to be connected to separate measuring circuits. One of these circuit-s is provided --through electrodes 51 and 52 which are directly connected to galvanometer 9 `by leads 55 and S6 to provide la high external resistance circuit in the same manner as `the apparatus shown in Fig. 2. At the same time the secondgpotenti-al measuring circuit is provided lby electrodes S3 and 54 which are connected in a very low Vresistance circuit to iield coil 57 of an alternator driven at constant speed by motor M-l for the same purpose jas described in connection with Fig. 5. However, in the present apparatus the output of armature 58, which varies in accordance with the potential existing lbetween the electrodes connected to iield coil S7, is applied to slip rings 61 and then connected to the primary winding of a high impedance transformer 62. The purpose of this latter arrangement is `to provide la more satisfactory coupling arrangement to the input of amplifier 64, since the input impedance of amplifier (ad can tbe better matched to the impedance of secondary winding 63 of transformer 62 `than to the output of the alternator. However, since it is desirable to transmit a D. C. signal to galvanometer 66 at the surface, in `order to remain free 4of the inductive and capacitive effects between the cable leads, a rectifier is preferably interposed between the output of amplifier 64 and cable leads 67 and `68. Y

Since it has been found that the internal resistance of the generating source of the spontaneous potential produced in a well bore `bears a relationship to the physical characteristics and particularly to the porosity yof the formations traversed by a well bore, a novel method has been provided by the present invention for qualitatively differentiating the porous and non-porous `strata in a well.

While the apparatus shown in Figs. l to 4 have -been described only as applied to the study of spontaneous potential-s, it will be apparent to those skilled in the art that resistivity measurements may be made with such apparatus in the conventional manner. Likewise, the adapt-ation of presentconventional electrical logging equipment well known in the art to'my novel method may |be made without ldeparting from the teaching `of my invention.

Since it is wellA understood in the art that current and Vpotential yare interrelated inV the measurement of either value, when the terms spontaneous current, or potential, are used inthe claims I intend to include the complementary measurable quantity since whether current or potential is measured, is merely a matter `of choice by the operator of the measuring circuit.

While various modifications and additions to the invenaan inclusi-:anemia will `occur to those Skilled in the art, all such modifications and additions in both the vapparatus and the method as fall within the scope of the appended claims are hereby included, and I therefore intend to limit the scope of the invention only within the terms of said claims.

I claim: l

1. Apparatus for logging the internal resistance of a generating source for natural potentials existing along a well bore comprising a sealed housing member having an electrically insulated wall member and supported for traversing said well bore on a multi-conductor well logging cable, at least a pair of longitudinally-spaced electrodes disposed on the outer surface of said wall member, a tirst measuring circuit including a predeterminable load connected between said pair of electrodes for determining the current flow therebetween and lthrough said predeterminable load due to natural potentials present in the well bore being applied across said electrodes, means for generating a first measuring potential variable in magnitude in response to the magnitude of said current iiow through said predeterminable load, a second measuring circuit including a different predeterminable load connectable to said electrodes, means for generating a second measuring potential variable in magnitude in response to the magnitude of current flow through said second predetermnable load, and means including said logging cable for correlating said rst and second measuring potentials generated by the same natural potential sources with the depth of said housing within the well bore.

2. Apparatus in accordance with claim 1, including switch means for connecting different loads between Asaid pair of electrodes to form said first and second measuring circuits and thereby provide at least two different current ilow conditions between said electrodes for the same natural potential source, and means for correlating the two measuring potentials produced by the same potential source with the depth of said housing within the well bore.

3. Apparatus in accordance with claim l including a second pair of longitudinally-spaced electrodes mounted upon the outer surface of said wall member, the spacing between the electrodes of each pair being substantially equal, said second measuring circuit having said second predeterminable load connected in series with said second pair of electrodes for determining the difference in current ow 'between the {inst-named pair of electrodes and said second pair due to the same natural potential being applied across dilierent loads and said correlating means including means for recording the difference in said current flows in accordance with the depth of said natural potential sources within said well bore.

4. Apparatus for electrically logging a bore hole comprising a housing member adapted to Ibe lowered into said bore hole comprising an elongated, tubular wall member having its ends closed to form a sealed chamber, at least a pair of longitudinally spaced potential sampling electrodes disposed on the outer surface of said wall member `and insulated therefrom, electrical insulating means extending longitudinally to cover at least said outer surface of said tubular wall member intermediate each of said electrodes, and poten-tial responsive circuit means positioned in said housing member and connected between said electrodes, said circuit means comprising an alternating current generator means having a field winding connected in series with I-said electrodes, motor means for driving said generator means at substantially constant speed to produce an alternating current output from said generator proportion-al to the natural potential current iiowing between vsaid electrodes and through said field winding, and rectifier means connected to the output of said generator means, and a measuring circuit connected to the output of said rectifier means whereby the rectified 10 voltage may be measured as yav direct indication ofthe natural potential current flow 'between said electrodes.

5. Apparatus for electrically logging a bore hole to determine the internal resistance of the generating source of the natural potentials existing therein comprising a housing member adapted to be lowered by a well logging cable into said bore hole, said housing member comprising an elongated, tubular wall member having its ends closed to form a sealed chamber, at least a pair of longitudinally spaced potential sampling electrodes disposed on the outer surface of said wall member and insulated therefrom, electrical insulating means extending longitudinally to cover at least said outer surface of said tubular wall member intermediate each of said electrodes, and potential responsive circuit means positioned in said housing member and connected between said electrodes, said circuit means comprising A. C. generator means having a field winding connected in series with said electrodes, motor means for driving said generator means at substantially constant speed to produce an A. C. output from said generator proportional to the natural potential current owing between said electrodes and through said field winding, a plurality of resistances having one terminal connected in common to one of said electrodes, switch means for selectively connecting the opposite terminal of each of said resistances to said field winding, rectifier means connected to the output of said generator, and a measuring circuit connected through said cable to the loutput of said rectifier means whereby the rectified voltage may be measured as a direct indication of the natural potential current flow between said electrodes under at least two load conditions for said generator means.

6. Apparatus for electrically logging a bore hole to determine the internal resistance of the generating source of the natural potentials existing therein comprising a multi-conductor well logging cable, a housing member supported by said cable and adapted to be lowered thereon into said bore hole, said housing member comprising an elongated, tubular wall member having its ends closed to form a sealed chamber, at least two pairs of longitudinally spaced potential sampling electrodes disposed on the outer surface of said wall member and insulated therefrom, electrical insulating means extending longitudinally to cover at least said outer surface of said tubular wall member intermediate each of said electrodes, potential responsive circuit means positioned in said housing member and connected between one of said pairs of electrodes, said circuit means comprising A. C. generator means having a eld winding connected in series with, said pair of electrodes, motor means for driving said generator means at substantially constant speed to produce an alternating current -output from said generator which is proportional to the natural potential current flowing between said pair of electrodes and through said eld winding, transformer means having a primary winding connected to the output of said generator, amplifier means connected to the secondary winding of said transformer, and rectifier means connected between said amplifier means and a potential measuring circuit, said potential measuring circuit including at least one of the conductors of said multi-conductor well logging cable for indicating at the earths surface the natural potential current flow between said one pair of electrodes under a low resistance loa-d condition, another measuring circuit including the distributed resistance of at least another of said conductors of said cable connected directly to the other of said pair of electrodes to provide an indication of the natural potential current ow under a high resistance load condition, and means for indicating the natural potential current flow of the same vertical interval in the well bore under both said low resistance and high resistance load conditions.

7. Apparatus for logging the internal resistance of a generating source of natural potentials existing along a 11 Weil bore,V comprising a `seaied hosing adapted to travel-s the weliborc While suspended' by al multi-conductor Wil logging' cble, t iaf; a @elif f ingitudiiialeiy' Spacd eitld mfd "exflliiy ii Said hus'iiag Viifii'- sulated thefe'from, eiectrieai insulating means extend-ing longitudinally to'- eeveri at leest said eute'i surface ef said tubular Walim'em'he irtifnedite eah Gi Vsaid' elcfoe-s, a cunent measuring `eifeuit within seid housing connected between said electrodes, said. i-rcuit including a 'iieid Winding for a potential uxg'gertlating means eonnecied in seris Wifh sidi eictfrode's, mefnsiiiciil'ding said eld winding fdr prdciig ani A. C. output'- ffom said patentiai. generating mesnil variable `ini magnitude in accordance with. the curifent ioW in- Said current measuring circuit due til Saidv nt'i'al potential between said' e1ec trades, means for s'elctilviy Varying the Ot-airesistance 12 ofvsad entrent niearng' circuit, ineens for rectifying sid A: C-. utpiit to plvide direct cuirent' indicating voig, and means for oi'r'eitiin'g variations in said direcvt'cilrrnt indicating vt-g iiidr at least two dierent Yconditions of total resistance over the same interval of weil bore with the depth yof said housing in said Well bore as aY measire f th variations in the internal resistance of the generating sources' of said natural potentials.

Reerences Cited inthe file of this patent UNITED STATES PATENTS 

